Drying drum



Oct. 14, 1941. R. E. CLEVELAND 2,259,024

DRYING DRUM Filed March 28, 1940 Patented Oct. 14,v 1941 DRYING DRUM Ralph E. ClevelanmWaten-bury, Conn.

Application March 28, 1940, Serial No. 326,483

` (ci. csr-sv) 'i claims.

This invention relates to drying drums, and it comprises atube or drum of heat conducting metal, such 'as a drying drum, for example, one

surface ofwhich is heated by a vapor whichl normallytends to form a heat-insulating lm of condensate on the metal surface; said tube or drum having a smooth coating of chromium on the surface exposed to said vapor, whereby the vapor is condensed on the chromium surface in the form of drops rather than in the form of a heat-insulating lm; means being provided for wiping said chromium surface and usually for removing condensed vapor from said surface, the metal surface opposite that exposed to said vapor being also advantageously coated with chromium, whereby the over-all heat transfer, corrosion resistance and wear-resistance is increased; all as more fully hereinafter set forth and as claimed.

Several different types of vapor-heated, rotary drying drums are in commercial use. In one type the interior of the drum is heated by steam, the material to be dried being applied to the exterior surface of the drum at one point and the .dried material being removed by Scrapers at another point, during rotation of the drum. In another type the drum is supplied with an exterior steam jacket, the material to be dried passing through the interior of the drumwhich is usually mounted at a slight angle to the horizontal. These prior art drums have usually been constructed of steel, cast iron or bronze.' Drying drums of these types are usually heated by steam its sensible heat as well as its heat of vaporizaf f tion. Pools of condensate are usually formed at the lowest points of the heating chambers. These pools of water tend tofollowY the drum surfaces during rotation and to form films of water covering the entire heat-exchanging surequipmentr in which condensation of vapor oc.

curs on the heat-exchanging surfaces.

I have found that in all prior art equipment the case of rotary equipment, heat-insulating films of water are usually formed over the heatexchanging surfaces which substantially reduce the heat efiiciencies of the equipment. `While the heat conductivity of water isvof the order of porcelain, the thicknesses of the films which are formed would be expected to be so small as to produce a negligible effect upon the over-al1 heat conductivity. Perhaps this is the reason why the important effect of these films upon the heat eciencies of drying drums and the like has been previously overlooked. But at least no practical method has hitherto been suggested of eliminating these films and thereby increasing heat eiilciency.

I have found that, if the metal surface which is exposed to the heating vapor in equipment of the described type is of chromium, the vapor tends to condense on the chromium surface in the form of drops rather than in the form of a continuous film, thereby producing a substantially increased heat conductivity. Moreover, if

theheating -surface is subjected to a continuous wiping or cleaning action, it will be found to be from about 85 to 95 per cent free from condensate. This is true in the case of heating vapors, such as diphenyl or Dowtherm, for example, as well as in the case of steam. It is also advantageous and, if the drum is constructed of a" soft material such as copper, essential to employ a coating of chromium on the drum surface which is opposite that exposed to the heating vapor since this increases the over-all heat transfer and corrosion resistance. And, in the case of drying drums in which the dried materialis removed'from the outer surface by means of Scrapers, the use of a coating of chromium on this surface is advantageous for the reason that a smoother, harder surface is formed which is less subject to corrosion and scoring by the Scrapers. The advantages of having the exterior surface of this type of drum plated with chromium has, of course, been recognized previously and chromium plated dryin'g drums are in common use. But it is of considerably greater importance to have the inner surfaces, which are exposed to the heating vapor, of chromium,

.as pointed out previously.

My invention can be explained in more detail by reference to the accompanying drawing which shows,' more. or less diagrammatically, a preferred embodiment of this invention as applied to drying drums. In this showing, the iigure of the described type, but more especially in is a vertical section through an internally heated drying drum having an inner and outer surface of chromium.

The figure is supplied with descriptive legends which are believed sufficient to show the novel construction of the present invention. The drying drum proper. having an inner heating chamber, is shown at I. This drum is provided with a stationary hollow axle which is used for withdrawing condensate which is collected by the scoop 3 in the conventional manner, the hollow axle forming a bearing for the drum as usual and the condensate being forced out by pressure. A wiping or cleaning means. such as a squeegee or doctor blade 4, is provided in this drum to keep the inner surface oi' the heating chamber clean. This wiping means can be mounted on the central tube as shown. It is possible, of course, to have the central tube! of this drum rotated in a direction opposite the rotation of the drum but this is usually not required. Both the inner and outer surfaces of this drum are of smooth chromium as indicated by the descrip-l tive legends.

The chromium surfaces of this invention can be produced in any of the conventional ways, such as by electroplating. It is also possible to pack the inner surface of a drum with a mixture of salts and chromium powder, the outer surface of the drum then being heat treated in order to forml an inner alloy surface layer of high chromium content. It is also advantageous to heat treat an electroplated coating of chromium in such fashion that an alloy is formed with the metal of the drum. A third method of providing the desired chromium surface is to solder into the drying drum a shell of metal which has been previously4 chromium plated. The most convenient method. of course, is to electroplate chromium directly upon the surface. But no matter what method is used of obtaining the high chromium content layer, it is necessary that the finished surface be smooth and clean. If the electrodeposited surface, for example, is not sumciently bright and drying drums constructed'in this manner have drying efficiencies which are substantially greater than that of any drums which are now in commercial use. And they have equal strength and wear resistance.

With the use of the present invention drying drums can be built which produce the same amount of dried product but which are so much smaller than comparable prior art drums that a substantial saving in initial cost is realized. 'I'hese smaller drums occupy less floor space and cost less to operate owing to their higher heat efflciencies and lower cost of care and upkeep. With drying drums of equal size, the drums constructed in accordance with the present invention are capable of substantially increased rates of production at higher efficiencies.

In one comparative test between a drying drum constructed in accordance with the present invention and a commercial prior art drum, condensed skim milk was used as the material to be dried. The atmospheric drum drier used in 1 this test was constructed with two Iadjacent drums smooth it may be polished by any of the known methods.

The dryingY drums of this invention may be constructed of any desired heat-conducting metal. High purity copper is especially advantageous owing to its high heat conductivity. Unplated copper has been but seldom used in drying drums because of the softness of its surface which is easily damaged by the rubbing of a doctor blade. Moreover in many cases the material to be dried has a corrosive effect on copper, with the result that the surface is further roughened and the product contaminated. As a consequence the dried material tends to adhere to the drum surface and to thereby reduce heat transfer. Steam tends to condense on copper surfaces inthe form of a nlm and this film has such a low heat conductivity that the over-all heat transfer is reduced to such an extent that the additional cost of copper drums is not usually justified. By the use of the present invention, however, it is possible to construct copper drums without any of the disadvantages mentioned. I am enabled to V obtain the full advantage of the higher heat conductivity of the copper because the water film is eliminated together with its heat-insulating effect. The use of a scraper to wipe the inner chromium surface maintains this favorable condition. By also chromium plating the exterior of the drum I avoid the above-described diiculties obtained when copper of high purity is used on rotating at the same speed but in opposite directions, the milk being fed to the space between the drums. One of the drums was a conventional drum of unplated cast iron and the other was a copper drum chromium plated both outside and inside in accordance with the present invention and provided with an interior cleaning element.

In this test 130 pounds of condensed skim milk, havinga density of 19 B. at 140 F., were fed into the machine. 24 pounds of milk powder containing 4 per cent moisture were produced on the chromiuml plated roll while only 18 pounds were produced on the cast iron drum, this having a moisture content of 9 per cent. The chromium plated copper roll therefore produced one-third more powder with a water content of less than half of that produced on the cast iron drum. The condensate from the chromium plated roll weighed 6l pounds while that from the cast iron roll weighed 52 pounds, a difference of about 18 per cent. Thechromium plated roll .therefore not only had a greater capacity, in output per unit of time, than the cast iron roll, but it also operated at a higher heat eiciency, since the steam consumption was not proportionally greater. Steam pressures during this test ranged from 40 to 60 pounds per square inch and the speed of rotation was varied between 14 to 20 R. P. M. The maximum efficiency of the chromium plated roll could not be obtained in this test for the reason that it was necessary to operate it at the same speed as the cast iron roll. It would doubtless have been possible to have obtained a greater efficiency had it been possible to vary the steam pressure and rate of rotation to the optimum values for the chromium plated roll.

While the above test does not give strictly comparable results owing to the use of copper instead of cast iron in the chromium plated roll, my previous experience with unplated copper rolls has shown that but little increase in emciency would be expected by the use of these rolls in comparison with cast iron rolls, owing to the formation of a heat insulating iilm of water oni the unplated copper. The increase in heating efficiency and production obtained in the above test is therefore believed to be highly surprising. 'The use oi' an unplated copper roll would not be feasible in the operation of producing milk powthe outer surface of a drying drum. Copper der. as above, owing to contamination from the copper and rapid wear of the surface of the drum by the doctor blades. It is therefore evident that modiilcations can be made in'the specic constructons described without departing from the purview of this invention. My invention is obviously applicable to the case of any heating operation in which heat exchange takes place between a vapor, which upon condensation tends to form a heat-insulating nlm, and a material to be heated or dried, the vapor and the material to be heated or dried being separated by a heat exchanging wall. This includes all types of heat exchangers of the closed type, such as evaporators, steam heated catalytic chambers, feed water heaters etc. Drying drums, of course, can be considered as one type of heat-exchanging equipment. The heat exchanging wall maybe constructed of any metal which is suihciently heat conducting but it is advantageous to employ a metal which can be directly chromium plated. Intermediate layers of copper or nickel can be employed according to conventional chromium plating practice. The cleaning element or blade, which is employed to keep the chromium surface clean, can be made of any suitablematerial such as rubber, neoprene, Bakelite or other plastic or metal such as brass or bronze. It is advantageous to have this blade exible. Other modiilcations which fall within the scope of the following claims will be immediately evident to those .skilled yinthisart.

ber at least partly formed by said shell of metal and heated by a vapor under conditions tending to produce the condensation of a heat-insulating nlm of condensate on a surface ofV said metal shell, a coating of chromium over the said surface of said metal shell whichvisexposed to said vapor and means for continuously wiping said chromium coating, whereby. the vapor is condensed in the form of drops rather than in the form of a heat-insulating film thereby substantially increasing the heat lefficiency of said drying drum.

2. A drying drum comprising a cylindrical shell of metal adapted to `be rotated, a central heating chamber formed by said shell oi metal heated by a vapor tending to condensewith the formation of a pool of condensate at the bottom of said chamber and a ilm of condensate over the interior of said metal shell, a smooth coating of chromium over said inner surface and means for continuously wiping said chromium coating, whereby the said vapor condenses in the form of drops rather than in the form oi a lm and the heat eciency of said drying drum is substantially increased.

3. 'I'he drying drum of claim 2 wherein means are also provided for removing said pool of condensate.

4. The drying drum of claim 2 wherein the outer surface of said drum is also coated with smooth chromium.

5. A drying drum comprising a cylindrical copper shell adapted to be rotated, the inner and outer surfaces of which are coated with a smooth coating of chromium, and having an inner heating chamber fo'rmed by said copper shell heated at least partly by the condensation of a vapor on the' inner surface of said shell and means for continuously wiping the chromium-coated inner wall of said shell. Y

6. The-drying drum of claim 5 wherein means are also provided for removing condensate from said heating chamber.` Y

7. The drying drum oi claim- 5 wherein a squeegee is provided for wiping the chromiumcoated inner wall of said copper shell.

RALPH E. CLEVELAND. 

